U.S. patent application number 16/323572 was filed with the patent office on 2019-06-06 for method for preparing a sample for laser induced breakdown spectroscopy.
This patent application is currently assigned to FOSS Analytical A/S. The applicant listed for this patent is FOSS Analytical A/S. Invention is credited to Daniel ADEN, Thomas NIKOLAJSEN.
Application Number | 20190170617 16/323572 |
Document ID | / |
Family ID | 56940110 |
Filed Date | 2019-06-06 |
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United States Patent
Application |
20190170617 |
Kind Code |
A1 |
NIKOLAJSEN; Thomas ; et
al. |
June 6, 2019 |
METHOD FOR PREPARING A SAMPLE FOR LASER INDUCED BREAKDOWN
SPECTROSCOPY
Abstract
A method for preparing a sample of organic material for laser
induced breakdown spectroscopy (LIBS) may include obtaining
granular organic material, forming a portion of the granular
organic material into a sample pellet, and searing the organic
material. The searing may include searing only an exposed end
surface of the sample pellet on which LIBS analysis is to be
performed. The method may include pressing the seared sample pellet
to consolidate the material comprising the seared end surface.
Inventors: |
NIKOLAJSEN; Thomas;
(Hilleroed, DK) ; ADEN; Daniel; (Hilleroed,
DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FOSS Analytical A/S |
Hilleroed |
|
DK |
|
|
Assignee: |
FOSS Analytical A/S
Hilleroed
DK
|
Family ID: |
56940110 |
Appl. No.: |
16/323572 |
Filed: |
September 8, 2016 |
PCT Filed: |
September 8, 2016 |
PCT NO: |
PCT/IB2016/055342 |
371 Date: |
February 6, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 2223/618 20130101;
G01N 33/4833 20130101; G01N 2223/312 20130101; G01N 1/286 20130101;
G01N 21/718 20130101; G01N 2223/6126 20130101; G01N 2001/2886
20130101; G01N 1/44 20130101 |
International
Class: |
G01N 1/28 20060101
G01N001/28; G01N 1/44 20060101 G01N001/44; G01N 21/71 20060101
G01N021/71; G01N 33/483 20060101 G01N033/483 |
Claims
1. A method for preparing a sample of organic material for laser
induced breakdown spectroscopy (LIBS), the method comprising:
obtaining granular organic material; forming a portion of the
granular organic material into a sample pellet; and searing the
granular organic material.
2. The method according to claim 1 wherein the searing the granular
organic material includes searing only an exposed end surface of
the sample pellet to be subjected to a LIBS analysis.
3. The method according to claim 2, further comprising: pressing
the sample pellet, subsequently to searing only the exposed end
surface of the sample pellet.
4. The method according to claim 1, wherein the forming the portion
of the granular organic material into the sample pellet includes
pressing the granular organic material based on applying pressure
to the granular organic material to form a consolidated mass as the
sample pellet.
5. The method according to claim 1, wherein the granular organic
material includes plant material.
6. A method of performing laser induced breakdown spectroscopy
(LIBS), the method comprising: preparing a sample of organic
material according to the method of claim 1; directing a laser beam
pulse to a seared surface of the sample of organic material to
produce a plasma ablation event; and performing a spectrometric
analysis of light emitted from plasma generated in the plasma
ablation event to identify constituent elements of interest in the
sample of organic material by their characteristic emission
wavelengths.
7. The method according to claim 1, wherein the searing the
granular organic material is prior to the forming the portion of
the granular organic material into the sample pellet.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a National Stage of PCT/IB2016/055342,
filed on Sep. 8, 2016, the entire contents of which are
incorporated herein in their entirety.
BACKGROUND
[0002] The present invention relates to a method for preparing a
sample for laser induced breakdown spectroscopy (LIBS) analysis and
in particular to a method of preparing a sample of solid organic
material for LIBS analysis.
[0003] LIBS is a spectrochemical technique that uses a pulse laser
of very short pulse duration (typically between nanoseconds and
femtoseconds) which is focused on a sample to create transient
temperatures upwards of 10,000 Kelvin.
[0004] In this environment, a portion of the sample is converted
into plasma and the chemical bonds are broken to produce
electronically excited atoms and ions. These excited species give
emit radiation at specific wavelengths that depend on the
constituent element.
[0005] By analysing the light emitted by the plasma it is possible
to identify the constituent elements of interest by their
characteristic emission wavelengths and to measure the
concentration of the constituent elements of interest by measuring
the intensity of the light at their characteristic emission
wavelengths.
[0006] Ideally, the intensity of the characteristic emission
wavelength is dependent only on the quantity of the associated
constituent element present in the sample. However, it is generally
known that variations in sample properties, being variations in
physical or chemical properties of the matrix in which the
constituent elements are found, affect the intensities of the
characteristic emission wavelengths of the associated constituent
elements. This problem, generally referred to as "matrix effects",
is well known in the art and is a factor that limits LIBS
accuracy.
[0007] To be properly analyzed using LIBS the samples, among other
things, should be homogeneous. Typically samples of naturally
occurring organic material, such as plant material samples, are not
naturally homogeneous, thus they must be processed into a
homogeneous sample. To achieve this, the organic material is first
broken down into unconsolidated particles or granules, usually by
being ground, shredded or pulverized, and then the granular sample
is converted into a single solid unit by forming the granular
material into a sample pellet. Forming is typically done by
pressing the particulate material into a consolidated unit, by
mixing an epoxy or other binder with the sample and curing to form
the sample pellet or a combination of both pressing and adding a
binder.
[0008] However, even using extremely fine powder inhomogeneities in
the organic matrix of the sample may often remain. As the size of
the granules increase so often does the inhomogeneity of the final
sample.
[0009] It is known from U.S. Pat. No. 7,663,749 to provide a LIBS
system for measuring a quantity of a constituent element of an
inhomogeneous sample in which a division of the sample into domains
is made using an image acquired by a CCD camera. LIBS analysis then
performed for each domain and the concentration of an element of
interest for a sample is calculated from the LIBS analysis of each
domain and the relative volumes of the domain and the whole sample.
By limiting the analysis to various discrete domains any matrix
effects are reduced and the overall accuracy of the concentration
analysis is improved.
[0010] Other LIBS systems, such as the one described in U.S. Pat.
No. 8,319,964, employ a first laser to ablate the sample and a
second laser to perform LIBS analysis of the ablation plume. This
LIBS analysis is typically performed at a location away from the
ablation site and a carrier gas transports the ablation plume to
the site for LIBS analysis. As the LIBS analysis of the plasma is
uncoupled from the ablation event any matrix effects should be
reduced.
[0011] Such systems are typically more complicated to construct and
are thus more expensive.
SUMMARY
[0012] It is an aim of the present invention to mitigate matrix
effects on LIBS analysis of solid organic material.
[0013] According to a first aspect of the present invention there
is provided a method for preparing a sample of plant material for
laser induced breakdown spectroscopy (LIBS) comprising the steps of
obtaining granular organic material; and forming at least a portion
of the granular organic material into a sample pellet, such as by
press-forming the granular organic material into a consolidated
mass; wherein the method further comprises the step of searing the
organic material.
[0014] In some embodiments the step of searing is performed only on
a surface of the sample pellet to be analysed using LIBS.
[0015] Searing (also known as charring) of the sample surface
induces a thermochemical decomposition of the organic sample
matrix. For a subsequent LIBS analysis, the thermochemical
decomposition has two effects; 1) the emission lines for minerals
are stronger since the elements are more easily ionized; 2) the
accuracy for a quantitative elemental abundance analysis is
improved since the seared matrices, when comparing different plant
materials, have more in common with respect to chemical composition
than have the un-seared matrices.
[0016] Usefully, but not essentially, the sample pellet is
press-formed and re-pressed after such searing of the surface. This
provides a compacted, relatively flat, surface for LIBS
analysis.
[0017] According to a second aspect of the present invention there
is provided a method of performing laser induced breakdown
spectroscopy (LIBS) comprising the steps of preparing a sample of
organic material according to the method of the first aspect of the
present invention; directing a laser beam pulse to a seared surface
of the sample of organic material to produce a plasma ablation
event; and performing a spectroscopic analysis of light emitted
from plasma generated in the plasma ablation event to identify
constituent elements of interest in the sample of organic material
by their characteristic emission wavelengths.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and other advantages and features will be better
understood from a consideration of the following description of one
or more exemplary embodiments of the method and the system
according to the present invention made with reference to the
drawings of the accompanying figures, of which:
[0019] FIG. 1 shows a flow chart illustrating an embodiment of the
method of the present invention;
[0020] FIGS. 2A-2D show Comparative LIBS spectra of seared and
un-seared samples of Feed and Soy; and
[0021] FIG. 3 shows Comparative LIBS spectra of samples of Feed
under different searing durations.
DETAILED DESCRIPTION
[0022] An illustrative embodiment of the method according to the
present invention will be described with reference to FIG. 1.
[0023] A first step 4 of the method 2 generally consists of
obtaining granular organic material. In a particular embodiment a
sample of plant material (being material from the plant itself or
material manufactured using such material, such as animal feed,
flour or foodstuff) is processed into unconsolidated granules, for
example by shredding, grinding or pulverizing the material. This
processing may be achieved manually, for example using a mortar and
pestle, or mechanically, for example using a grinder or shredder,
and may be done optionally after the material is dried.
[0024] A second step 6 generally consists of forming at least a
portion of the granular material obtained at the first step 4 into
a sample pellet. In a particular embodiment the granular organic
material, for example plant material, is loaded into an open ended
cylindrical die and pressure is applied to the material in order to
form a sample pellet of consolidated granular plant material which
is preferably retained in the die to help maintain structural
integrity and to enhance ease of handling. The so-formed pellet
presents an exposed end surface, at which LIBS analysis will be
performed. A conventional hydraulic laboratory pellet press may be
employed to form the sample pellet.
[0025] A third step 8 generally consists of searing the granular
organic material. In an embodiment this step is performed after the
step 6 of forming the sample pellet and involves applying heat only
to the exposed end surface of the pellet. In an alternative
embodiment the step of searing 8' may be performed on the organic
material before the step 6 of forming, for example after the step 4
of obtaining the granular organic material.
[0026] A fourth step 10 is performed in embodiments where the step
8 of searing is performed after the step 6 of forming a sample
pellet. This fourth step 10 generally consists of pressing (or
re-pressing) the sample pellet. At this step 10 pressure is applied
to the sample pellet in order to consolidate the material which
forms the seared exposed end surface of the sample pellet. The
press employed at the step 6 of forming is conveniently employed at
this step 10 of pressing (or re-pressing) the sample pellet.
[0027] Samples are prepared according to the method (2) described
above in respect of FIG. 1 by firstly obtaining granulated plant
material (4); then press-forming the granulated plant material into
a sample pellet (6) having an exposed end surface for LIBS
analysis; then searing the exposed end surface (8); and finally
re-pressing the seared exposed end surface (10) before presenting
the sample for LIBS analysis. LIBS analysis is performed on the
seared exposed end surface of each of the resulting sample pellets.
Essentially, this is achieved by directing a laser beam pulse to a
seared surface of the sample of organic material to produce a
plasma ablation event and performing a spectrometric analysis of
light emitted from plasma generated in the plasma ablation event to
identify constituent elements of interest in the sample of organic
material by their characteristic emission wavelengths. Spectra
generated by the spectrometer of the LIBS system for seared (solid
lines) and un-seared (broken lines) sample pellets are illustrated
in FIGS. 2A-2D for sodium (Na) in mixed ration feed (FIG. 2A); for
Calcium (Ca) in mixed ration feed (FIG. 2B); for potassium (K) in
soy (FIG. 2C) and for phosphorus (P) in soy (FIG. 2D). As can be
seen the spectral features associated with the elements in each of
the samples are all enhanced in the seared sample pellets.
[0028] The effect of searing duration on LIBS spectra from the
seared exposed end surfaces of sample pellets of mixed feed ration
produced according to the method of FIG. 1 is illustrated in FIG. 3
for calcium (Ca) and includes the step 10 of re-pressing the sample
pellet after searing. Five tons per square centimeter is applied to
the plant material in the die for ninety seconds at each pressing
stage 2, 10. LIBS analysis is performed at five different searing
levels: (a) `No searing` (i.e. the method according to FIG. 1 is
performed up to and including the step 6 of press-forming
particulate mixed feed ration into a sample pellet); (b) `Normal
searing` (i.e. searing is stopped when the exposed upper surface of
the pellet becomes black according to visual inspection); (c)
`Heavy searing` (twice the searing time compared to Normal
searing); and (d) `Heaviest searing` (three times the searing time
compared to Normal searing).
[0029] As can be seen from FIG. 3, the intensity of the LIBS
spectral signal due to Ca increases as the duration of the searing
increases (i.e. from (a) to (c)). The intensities begin to converge
as searing intensity increased and beyond a certain searing level
(between the Heavy (c) and Heaviest (d) searing durations) the
intensity of the LIBS spectral signal begins to reduce. Thus the
optimum searing time for a specific matrix and/or searing
temperature can be readily experimentally determined.
* * * * *